1. Field of the Invention
The invention relates to the construction and disassembly of roof reflectors for gas-cooled pebble-bed reactors.
2. The Prior Art
Reactor roof reflectors are known which consist of a plurality of graphite blocks arranged in close proximity to each other, suspended by means of tie rods from the upper part of the pressure vessel enclosing the nuclear reactor. A roof reflector of this type may consist, for example, of two layers of graphite blocks, wherein each of the blocks is equipped with a bore to allow for the passage of the cooling gas, the bores running in a broken line to provide shielding against neutrons. (See German Published Application DE-OS No. 15 64 186).
Another known roof reflector consists of a single layer of hexagonal graphite columns, the cooling gas passages provided centrally through the individual columns being shielded either by means of plates placed over the graphite columns or by helically wound stoppers inserted in the bores for the cooling gas (See French Pat. No. 14 26 264).
A roof reflector consisting of hanging columns is also known, the roof reflector being suspended from a thermal cover shield. Six each of the suspended columns define an opening for an absorber rod. The columns, arranged in three layers, are provided with slits for the passage of the cooling gas, the slits of adjacent layers being offset with respect to one another to provide shielding. (See German Published Application No. DE-OS No. 23 54 540).
All of the roof constructions described above are intended for use with gas-cooled nuclear reactors employing spherical fuel elements, the so-called pebble-bed reactors. Another roof reflector for a nuclear reactor of this type consists of a supporting plate fastened by means of threaded connections to the cover of the pressure vessel, and a plurality of graphite blocks arranged in at least two layers, a free space being provided between the supporting plate and the graphite blocks, the free space serving as a collector space for the cooling gas. (See German Published Application No. DE-OS No. 26 31 408). The graphite blocks are fastened to a retaining plate which closes off the lower boundary of the cooling-gas collector space and which is suspended by means of anchors from the supporting plate. The supporting plate and the retaining plate together constitute a static plate. Both consist of sections having overlapping junctions. Passages for the absorber rods are provided in the roof reflector. The flow of cooling gas passes through annular channels arranged coaxially in relation to absorber rods and possessing two different non-coincident, passage cross-sections. The cross-section of the upper passage is shielded in each case by means of an annular plate.